This invention relates to a method for producing a color filter and more particularly to a color filter advantageously employed as a color liquid crystal display device.
Among the currently employed methods for preparing a color filter, there are a dyeing method consisting in dyeing a transparent substrate with a binder containing dyes and pigments, a printing method and a pigment dispersion method.
Since the dyeing method consists in selectively forming a thin resin film on a substrate with dyes, a resist printing process and a photolithographic process need to be performed each time the color is changed. Although resist printing is unnecessary with the printing method, there is a limit to the refinement of color patterns and, the larger is the number of colors, the printing position becomes the worse. Although the fine color pattern is possible with the pigment dispersion method, a high precision photolithographic process needs to be performed each time the color is changed, resulting in a complicated process.
For overcoming the deficiency, there is proposed in Japanese Laid-open Patent Application No. 59-114572 (1984) a method for producing a color filter by an electrodeposition coating method. With this method, a transparent electrode is prepared by patterning a transparent electrically conductive film deposited on the substrate, the substrate is immersed in a colored electrodeposition bath for forming a colored layer by electrodeposition, and electrical voltage is applied only to a portion of the patterned transparent electrode which is to be dyed in the same color. The substrate is then immersed in a colored electrodeposition, and bath for forming a different color layer by electrodeposition electric voltage is then applied only to a portion of the substrate which is to be dyed in a different color. However, it is necessary with this method to perform a high precision patterning of the transparent electrode, and to pay meticulous care during the subsequent process not to break the fine pattern, because otherwise the subsequent coloring process is rendered difficult. Besides, the patterned transparent electrode needs to be electrically continuous, even in fine pattern sections, so that limitations are imposed on the degree of freedom of the pattern shape.
In Japanese Laid-open Patent Application No. 63-210901 (1988), there is proposed a method consisting in forming colored layers by light exposure, development and electrodeposition, using a mask having patterns only in areas to be dyed in the same colors and a positive type photosensitive resin composition, and repeating the steps of light exposure, development and electrodeposition a desired number of times. This method is inferior in stability because it makes use of a compound containing unstable quinone diazido groups. Besides, if the quinone diazido compound is brought into contact with an aqueous alkali solution, the quinone diazido compound in the unexposed part is also reacted with an aqueous alkali solution so that photosensitivity is markedly changed to present difficulties in the subsequent right exposure and development steps.
In these electrodeposition methods a transparent electrode for formation of colored layers is simultaneously used as an electrode for driving a liquid crystal. However, since the colored layers formed on the transparent electrode is made of an insulating material, the liquid crystal driving voltage becomes exceedingly high. For this reason, a transparent electrode for driving the liquid crystal is additionally provided on the colored layers formed in accordance with the above method for lowering the driving voltage. On the other hand, since the transparent electrode employed in the above method has a light transmittance of 80 to 85%, provision of two transparent electrode layers leads to lowered light transmittance to deteriorate the performance as a colored display substrate. For overcoming this defect, there is proposed in Japanese Laid-open Patent Application No. 1-22479 (1989) a method comprising forming a colored layer on a master plate and transferring it onto a transparent substrate. However, since the transfer is effected for each color with this prior-art method, it becomes necessary to achieve high precision alignment for each transfer operation, thus complicating the production.
On the other hand, in order to meet the demand for high performance of the device provided with a color filter, it has been desired to improve contrast and to prevent color purity from being lowered. In order to solve this problem, a method of forming a non-light transmitting film in a region of the color filter defined between neighboring pixels has been proposed. For forming the non-light transmitting film, there are known a method comprising forming pixels with alignment on a substrate on which a non-light transmitting film pattern is formed previously, and a method comprising forming a non-light transmitting film pattern with alignment on a substrate on which a pixel pattern is formed previously.
However, since it is necessary with these methods to effect an alignment operation between the pixel pattern and the non-light transmitting pattern, it is difficult with this precision to form a pattern of non-light transmitting pattern of a coincident size free of the light transmitting sections between the pixel patterns. If overlapped portions are produced, step differences are produced on a color filter, so that it becomes difficult to produce a color filter excellent in planarity.
With any of the above methods, high precision processing is required for alignment so that it is difficult to cope with the demand for a larger work size, that is a larger picture size with reduced costs.
In order to solve these disadvantages, a method for electrodepositing in turn each pixel has been proposed in Japanese Patent Application No. 3-43596. However, when black matrixes which light-intercepts the space between each pixel are formed in the same manner as each pixel, since black-hued coating containing carbon black having high light-interception is electrically conductive with a formed coating film, it is difficult to handle the black-hued coating similarly to the other colored coatings. Specifically, if the black-hued coating is formed followed by electrodepositing the other colored coatings, the other colored coatings are electrodeposited not only on the transparent electrically conductive layer but also on the black-hued coating film so that smoothness of the color filter is impaired. On the other hand, a method of electrodepositing a black-hued coating onto pixel free portions after the formation of the other pixels, lacks somewhat in precision to produce a black-hued light-intercepting layer requiring the highest precision.
To overcome these problems, there are proposed a method of using a black-hued coating containing mixed pigments other than carbon black, and a method of applying a photosensitive resin containing a black colorant to the overall surface of a color filter on which pixels other than the black pixel has been formed, subjecting only pixel free portions to light exposure to thereby cure photosensitive resin portions corresponding to the pixel free portions by exposing from the back side, i.e. substrate side to utilize each pixel as a light-intercepting portion, and developing and removing the photosensitive resin portions corresponding to pixel portions to form a black-hued coating film. However, in the former method black concentration is insufficient, while in the latter method the light-interception of each pixel is inherently insufficient so that it is impossible to obtain a satisfactory black-hued light-intercepting layer in either method.